Add logging output for `Job._schedule_next_run()` method

Currently the logging output for a scheduled job happens at the start of the `Job.run()` method. Ideally, this information should be logged when the job is scheduled rather than (or as well as) when the job is triggered, providing feedback that the job has been correctly scheduled. The output phrasing suggests that this should be the case as it gives a "next run" time but this is just the current time as the job is starting.